Light-transforming screen or reflector and method of making and using the same



UNITED STATES PATENT OFFICE-p HGHT-TRANSFORMING SCREEN 03 FLEOTOR AND METHOD OF MAKING AND USING THE SAME Gordon R. Fonda, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application October 8, 1936,

Serial No. 104,863

6 Claims. (Cl. 250-80) The present invention relates broadly to a lighttransforming screen or reflector wherein is utilized a new rhodamine derivative, specifically a rhodamine B compound in which an alkali metal B yields a fluorescence which is more permanent under exposure to light than rhodamine B.

It has been known that rhodamine and derivatives thereof may be incorporated in composiis substituted for hydrogen of the carboxyl group. tions that may be used for transforming light 5 The fluorescent properties of this compound for rays and, also, for adding light waves of definite light-transforming purposes may be utilized, for character to light from any source. Such comexample, in the manner described in my copositions are disclosed in the British Patent pending application Serial No. 60,276, filed Janu- 16,281 of 1911 to Peter Cooper Hewitt, and in m ary 22, 1936,.and assign d to h same assignee U. s. Patents 1,150,118, 1,888,655 and 1,188,777, 10 as h present v i Th p nt applicaalso to Hewitt. I make no broad claim to the use tion, which also includes method features, is a of rhodamine or any derivative thereof in comcontillllation-in-palt of s -P ing applicabination with other ingredients for such uses as tiOIl serial 60,276- In application Serial mentioned above, but I do claim as new and'novel 13 ,8 fi d March 1937, Which application an alkali metal salt of rhodamine B and its use in 15 is a division of the instant case, I have claimed particular compgsifions for ki 1ight-trans compositions comprising the above rhodamine formjng Screens or eflecto s having such compound. These compositions are especially proved permanence under exposure to light as to adapted for use in making the light-transformprovide a screen that is, in its practical effect,

: n Screen Claimed in the Present application difierent in kind from light-transforming screens 20 In the above-identified co-pending application h tof known to the are I have disclosed and claimed a fluorescent screen In the literature on the fluorescence f rhede or reflector of the type used for correcting the mine the form thereof commonly referred to is spectra of light sources which are deficient in Schultz 573 which is a chloride of a one or more spectral colors and the combination damjne B base It has the empirical formula 25 of such screens and light sources. In accordance CzeHhNZOaC; and the constitutional formula with that invention a fluorescent screen having a much longer life than attainable with similar 01 screens theretofore known and which is also highly efficient in converting impinging radiant 0 M 30 energy into fluorescent light of a complementary color is provided by employing a fluorescent coat- 0/ ing having a thickness in excess of that required to transform completely the impinging radiations, (CIHDOOOH but disposed in such a discontinuous fashion that The term rhodamine as used, gehera11y,here 35 the flnal blending of transformed and untransin refers to the form of rhodamine B m common formed light will afford a substantially proper use as a dye, that is in the chloride form Such spectral distribution. In the described co-pendform of rhodamine B also is known as rhodamine ing applicatim I 9 but did not 4 B hydrochloride. The rhodamine alkali metal 40 derivative of rhodamine B formed by substituting salt derivative embodying my invention has the 40 the alkali metal potassium, for hydrogen of the empirical formula CzaHaoNaQsClR and the com carboxyl group of rhodamine B, and the use of Stituhonal gormula this compound in solution form as a fluorescent m coating material for application to a Slll'ftCet to r form a light-transforming screen or re ec or.

40 In the following specification I shall describe M1199 with greater particularity alkali metal derivatives of rhodamine B, and how they may be made and used.

The present invention is based upon the (HD0003 50 observation, not heretofore known and which where R stands for an alkali metal. was not obvious from the literature describing In order that myinvention may be more clearly the properties of min B, that a rhodamine understood and practiced by those skilled in the dye in which an alkali metal has been substituted art to which this invention relates, the following for hydrogen of the carboxyl group of rhodamine specific examples are given as illustrative of 55 methods by which alkali metal salts of rhodamine B may be prepared. The processes will be described with particular reference to potassium,'but they are obviously applicable to the preparation of the rhodamine B salts of the other alkali metals, viz., lithium, sodium, rubidium and caesium.

Five grams of rhodamine B are dissolved in a solvent or mixture of solvents in which the potassium salt of rhodamine B is substantially more insoluble than rhodamine B. Examples of suittable solvents are acetone, ethyl alcohol, or mixtures thereof, or mixtures of either or both with water. Advantageously a :50 mixture of water and ethyl alcohol of about 95 per cent strength is employed. To such a solution is added a solution of a similar alcohol-water solvent containing an equi-molecuiar amount, and advantageously slightly more than the equi-molecular amount of potassium hydroxide required to form the potassium salt of rhodamine B, for example, 0.67 gram of potassium hydroxide. This mixed solution is evaporated until crystals appear. The crystals are separated from the mother liquor by suitable means, for instance, by vacuum filtration. The separated crystals are dissolved in a fresh mixture of alcohol and water and evaporated again until c y tals appear. The purified crystals are separated from the solution by vacuum filtration or other convenient meansand then are dried, for example, over phosphorous pentoxide.

Compositions for coating screens or other backing or supporting members may be made as hereinafter described, with solid alkali metal salt of rhodamine B prepared in the manner described and, for purpose of illustration, the potassium salt of rhodamine B is specified. A suitable amount, for instance, 2.07 grams of potassium salt of rhodamine B, is dissolved in one liter of an appropriate solvent such as ethyl alcohol. For use in a coating composition the alcohol solution of the potassium salt of rhodamine B is diluted to a ten-fold volume by adding a solution of cellulose acetate, advantageously a solution of cellulose acetate in acetone and containing, for example, about 30 per cent cellulose acetate. Such a mixture has a. concentration of the potassium salt of rhodamine B of 4(10)- molar, that is, one liter of solution contains a weight of the potassium salt which is 0.0004 times its molecular weight. This mixture when applied to a reflecting surface and dried shows, under exposure to light, a rate of fading that is surprisingly less than that shown by rhodamine B. Thus, a thin film composed of rhodamine B and a binder of cellulose acetate, when exposed to a quartz mercury-vapor lamp at a distance of 2% inches (4000 foot candles) showed a loss of about 75 per cent of its original fluorescence after 200 hours exposure and almost a complete loss of fluorescence after 400 hours exposure. A similar film composed of the potassium salt of rhodamine B and cellulose acetate, when exposed in a like manner, showed a loss of only 10 per cent of its original fluorescence after 1500 hours exposure.

Another example of a method of preparing an alkali metal salt of rhodamine B and a fluorescent coating composition containing such a salt, and again for illustrative purposes the potassium salt is mentioned, is as follows:

Equi-molecular solutions of rhodamine B and potassium hydroxide are prepared by separately dissolving 1.915 grams of rhodamine B per liter of a suitable solvent such as acetone or ethyl alcohol and 0.220 gram of potassium hydroxide per liter of the same-solvent as used with the rhoda mine B. These solutions are mixed and then diluted to a 5-fold volume by adding thereto a solution of cellulose acetate in a solvent which is miscible with the solvent used in making the mixed solution of rhodamine B and potassium hydroxide, advantageously a solution of cellulose acetate in acetone and containing, for example, about 30 per cent cellulose acetate. As in the previous illustration, one liter of the composition prepared as described contains a weight of the potassium salt of rhodamine B which is 0.0004 times its molecular weight.

For convenience in applying to light-reflecting surfaces a fluorescent coatingcomposition made in such ways as hereinbefore set forth, that is to say, in order to improve its brushing properties, various high-boiling diluents of the kind used in ordinary varnishes may be added thereto. Examples of such diluents are ethyl lactate, monomethyl ether of ethylene glycol and the acetate of the mono-ethyl ether of diethylene glycol ("Carbitol acetate). Advantageo'usly, I have added 10 per cent by volume of ethyl lactate to fluorescent coating compositions of the kinds above mentioned, to obtain a composition of improved brushing properties. Larger or smaller amounts of such diluents may be employed in order to improve the spreading and brushing properties of the composition. The composition may be applied to a surface without further dilution or, in certain cases, for example, when it is desired to form a very thin fluorescent film, the coating composition may be further thinned with a suitable solvent such as acetone or alcohol.

The intensity of fluorescence of a rhodamine dye is dependent not only upon the particular rhodamine dye but also upon the particular solid solvent or binder used in its application. I have found that greatest eiiiciency and life of a fluorescent fllm comprising the hereinbefore mentioned class of organic compounds is realized when such solid solvents or binders as cellulose acetate or Formvar are used. Formvar" is the trade name of a class of compounds produced, generally speaking, by reacting a partially or completely hydrolyzed polymerized vinyl ester with an aldehyde. The preparation of such compounds is set forth, for example, in U. 8. Patent No. 2,036,092, to Morrison et al. Binders other than cellulose acetate and "Formvar may be used, and may be of either organic or inorganic composition. Any suitable liquid solvent may be employed in applying the alkali metal salt of rhodamine B and binder therefor to the reflecting surface to be coated. Supporting members for the composition may comprise, for example, such substances as metals, for instance, aluminum, nickel, chromium, steel, etc., the surfaces of which are opaque and such as to produce diilused reflecticn; and such other materials, having the same opaque and diilusing characteristics described with reference to metals, as special glasses, porcelain, enameled surfaces, paper, cardboard, or silk, rayon, linen, cotton, or other textile fabric.

In preparing and using a coating composition containing an alkali metal salt of rhodamine B, it is important, in order to obtain maximum resistance of a film thereof to fading when exposed to light, that there be a definite relationship between said salt and binder as they are present in the dried film. This relationship can be most easily expressed by specifying the concentration of the alkali metal salt of rhodamine B in the liquid solution containing the preferred binderi Although I have found, taking the potassium salt of rhodamine B as a specific example, that best results are obtained when said salt is applied to a supporting member in the form of a solution containing, as a maximum, about 0.2 gram thereof per liter of solution, such preferred concentration is not essential in effectively utilizing the rhodamine salt of this invention for fluorescent purposes. The concentration oi? the salt may be substantially diminished, for example, to as low as about 0.02 gram of the salt per liter of solution, and for the same concentration of cellulose acetate solution (or solution of other binder), without marked effect upon the intensity of fluorescence and life of the dried fllm. In other words, for the same concentration of binder, advantageously I may use, in making a lighttransforming screen or reflector, a solution containing a weight of alkali metal salt of rhodamine B per liter of solution that is about 0.00004 to 0.0004 times its molecular weight. As pointed out in my co-pending application, Serial No. 60,276, filed January 22, 1936, a considerable increase in the efliciency of a fluorescent film is obtained if stripes of the fllm are applied with an optimum thickness, which, for a concentration such as indicated above as preferable, should be between about 2 and 14 mils.

Alkali metal salts of rhodamine B produced in accordance with this invention may 'be applied to a supporting or a backing member to form either a continuous or a discontinuous surface. They may be applied so as to form vertical, horizontal or other stripes, or the surface may be completely coated with the composition. They may be applied to a screen or reflector used in connection with a metal-vapor lamp, for instance, a mercury-vapor lamp, andin such use improves the quality of theillumination provided by such sources by producing the complementary color red, of which the light given oil by the ordinary mercury-vapor lamp is" almost entirely devoid. Alkali metal salts of rhodamine B, together with a suitable binder, may be applied'to a lighttransiorming screen adapted .both to transmit and to reflect light in the manner described with greater particularity in my co-pending application Serial No. 60,276, flled- January 22, 1936, and hereinbefore mentioned. They may also be used in making a light-transforming screen comprising a support and a composite coating thereon obtained by alternately applying to said support, first a solution containing the alkali metal salt of rhodamine B, a binder therefor, such as cellulose acetate or Formvar, and a liquid solvent; and then a solution of the binder and liquid solvent therefore. tions may be applied in either order, preferably the solution containing the rhodamine salt and Although the coating composibinder is applied. first and the solution of the binder is applied as the final coating.- The respective coatings are dried after each application by any suitable means, for example. under artificial heat or by exposure to the air. This method of making a light-transforming screen and the advantages of this procedure, viz., longer screen life, are set forth in more detail in the co-pending application of minepreviously mentioned.

i As an alternative procedure to that hereinbefore described for making a light-transformin screen and wherein a permanent supporting or backing member is employed, the coating composition comprising the alkali metal salt of rhodamine B and a binder therefor may be applied to a suitable temporary support, dried, and then stripped from the temporary support and used independently of any other support as a light-transforming or fluorescent screen.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A light-transforming screen -or reflector composed essentially of alkali metal salt of rhodamine B hydrochloride and a binder therefor.

2. A light-transforming screen or reflector comprising a supporting member and a coating thereon comprising alkali metal salt of rhodamine B hydrochloride and a binder therefor.

3. A- light-transforming screen or reflector comprising a supporting member and a coating thereon comprising alkali metal salt of rhodamine B hydrochloride and a product of reacting hydrolyzed polymerized vinyl ester-with an aldehyde.

4.'A light-transforming screen or reflector comprising a supporting member and a composite coating thereon consisting essentially of alternate layers of a mixture of alkali metal salt of rhodamine B hydrochloride and cellulose acetate, and of cellulose acetate.

5. A method of making a light-transforming screen or reflector which comprises coating a supporting member with a solution containing a weight of alkali metal salt of rhodamine B hydrochloride per liter of solution that is about 0.00004 to 0.0004 times itsmolecular weight, a

binder for said salt and a volatile solvent for said salt and binder, and drying the applied coating.

6. A method of making a light-transforming screen or reflector which comprises applying to a supporting member alternate coatings of a solution comprising alkali metal salt of rhodamine B hydrochloride, a binder for said salt, and a volatile solvent for said salt and binder, and of a solution comprising a binder for said salt and a solvent for said binder, and drying the applied coatings after each application.

GORTON R. FONDA.

k CERTIFICATE OF CORRECTION. Patent No. 2,119,992. March 7, 19 9.

GORTON R. FONDA. It is hereby certified that error appears in the above numbered patent requiring correction as follows: In the heading to the printed specification, line 5, name of inventor, for :Gordon R. Fonda" read Gorton R. Fonda;

page 1, second co1umn, l'ine 5, after the word "has" insert heretofore; line 9, for the British patent number "16,281" read 16,271; page 2, first column, line 38, before "potassium" insert the; and that thesaid Letters Patent should be read with this correction therein that the same may conform to the record or the case in the Patent Office.

Signed and sealed this 9th day of May, A. D. 1959.

. a Henry Van Arsdale (Seal) Acting Commissioner of Patents. 

